I. Field of Invention
This invention relates to a dietary approach to reducing the level of carcinogens in animals and their cells and thereby reducing the risk of developing cancer. In particular, this invention relates to the production and consumption of foods which are rich in cancer chemoprotective compounds. More specifically, this invention relates to chemoprotective compounds that modulate mammalian enzymes which are involved in metabolism of carcinogens. This invention relates to food sources which are extremely rich in compounds that induce the activity of Phase 2 enzymes, without inducing biologically significant activities of those Phase 1 enzymes that activate carcinogens.
II. Background
It is widely recognized that diet plays a large role in controlling the risk of developing cancers and that increased consumption of fruits and vegetables reduces cancer incidence in humans. It is believed that a major mechanism of protection depends on the presence of chemical components in plants that, when delivered to mammalian cells, elevate levels of Phase 2 enzymes that detoxify carcinogens.
Early studies on the mechanism of chemoprotection by certain chemicals assumed that these chemoprotectors induced activities of monooxygenases, also known as Phase 1 enzymes or cytochromes P-450. However, Talalay et al., [reviewed in "Chemical Protection Against Cancer by Induction of Electrophile Detoxication (Phase II) Enzymes" In: CELLULAR AND MOLECULAR TARGETS OF CHEMOPREVENTION, L. Wattenberg et al., CRC Press, Boca Raton, Fla., pp 469-478 (1992)] determined that administration of the known chemoprotector butylated hydoxyanisole (BHA) to rodents resulted in little change in cytochromes P-450 (Phase 1 enzyme) activities, but profoundly elevated Phase 2 enzymes. Phase 2 enzymes such as glutathione transferases, NAD(P)H:quinone reductase (QR) and glucuronosyltransferases, detoxify DNA-damaging electrophilic forms of ultimate carcinogens. Selective inducers of Phase 2 enzymes are designated monofunctional inducers. Prochaska & Talalay, Cancer Res. 48: 4776-4782 (1988). The monofunctional inducers are nearly all electrophiles and belong to 8 distinct chemical classes including (1) diphenols, phenylenediamines and quinones; (2) Michael reaction acceptors containing olefins or acetylenes conjugated to electron-withdrawing groups; (3) isothiocyanates; (4) 1,2-dithiole-3-thiones; (5) hydroperoxides; (6) trivalent inorganic and organic arsenic derivatives; (7) heavy metals with potencies related to their affinities for thiol groups including Hg.sup.2+, and Cd.sup.2+ ; and (8) vicinal dimercaptans. Prestera et al., Proc. Natl. Acad. Sci. USA 90: 2963-2969 (1993). The only apparent common property shared by all of these inducers is their ability to react with thiol groups.
Chemoprotective agents can be used to reduce the susceptibility of mammals to the toxic and neoplastic effects of carcinogens. These chemoprotectors can be of plant origin or synthetic compounds. Synthetic analogs of naturally occurring inducers have also been generated and shown to block chemical carcinogenesis in animals. Posner et al., J. Med. Chem. 37: 170-176 (1994); Zhang et al., Proc. Natl. Acad. Sci. USA 91: 3147-3150 (1994); Zhang et al., Cancer Res. (Suppl) 54: 1976s-1981s (1994).
Highly efficient methods have been developed for measuring the potency of plant extracts to increase or induce the activities of Phase 2 enzymes. Prochaska & Santamaria, Anal. Biochem. 169: 328-336 (1988) and Prochaska et al., Proc. Natl. Acad. Sci. USA 89: 2394-2398 (1992). In addition, these methods have been employed for isolating the compounds responsible for the inducer activities in plants and for evaluating the anticarcinogenic activities of these compounds and their synthetic analogs. Zhang et al., Proc. Natl. Acad. Sci. USA 89: 2399-2403 (1992) and Posner et al., J. Med. Chem. 17: 170-176 (1994).
Although inducer activity has been found in many different families of edible plants, the amounts are highly variable, depending on family, genus, species, variety, or cultivar of the plant selection and on growth and harvesting conditions. Thus, there is a need in the art to identify particular edible plants and methods of growing and preparing them that yield high levels of Phase 2 enzyme-inducer activity for chemoprotection. There is also a need for methods of growing and preparing edible plants that produce a known spectrum of specific inducers of Phase 2 enzyme activity in order to increase the efficiency with which specific carcinogens, or classes of carcinogens, are targeted for inactivation. In addition, there is a need for methods of plant breeding and selection to increase the level of Phase 2 inducer activity and to manipulate the spectrum of inducers produced in particular cultivars.